I'm curious how native and international English users feel about it.
I'd prefer "significance of my existence" because of the thing truehappiness pointed. Same thing can be said for Japanese young people; I guess more than 50% of them would say "what's that?" In addition to that, ayu isn't using "raison d'etre (or レーゾンデートル)" in her original text.
(Actually I used "significance of my existing" in my translation because ayu's original text is not "存在意義" but "存在する意義" and it can be parallel to "meaning of my being", but now I feel it may sound weird to English users.
haha)